1. Field of the Invention
The present invention relates to a technique for polishing a thin plate, in particular, to a polishing method which can polish a thin plate such as a semiconductor wafer (hereinafter, it may be simply referred to "wafer"), with a high flatness, and to a polishing apparatus.
2. Description of Related Art
In a process for fabricating a semiconductor integrated circuit, with the requirement of larger scale integration of recent semiconductor devices, the design rule for fabricating a device has a tendency to become finer and the depth of focus required for a stepper becomes shallower. Therefore, the requirement for the flatness of a semiconductor wafer which is the objective material becomes larger. Hereinafter, the term "flatness" means a uniformity with respect to variations in thickness of the wafer because use of a vacuum-chuck to hold the wafer in the stepper reforms a deformation of the wafer to some extent.
Generally, polishing of a wafer is carried out by pressing the wafer against a polishing pad while giving a relative motion between the wafer and the polishing pad and supplying an abrasive slurry. For example, such a polishing apparatus for a thin plate has a structure shown in FIG. 7.
In FIG. 7, the reference numeral 1 denotes a wafer which is a thin plate. The wafer 1 was processed to have upper and lower surfaces precisely parallel to each other and a uniform thickness, by slicing a silicon (Si) single crystal ingot by using an inner diameter saw slicing machine, thereafter by carrying out a lapping step and the like. A wafer or a plurality of wafers 1 are held on the lower surface of a holding plate 2. Both surfaces of the holding plate 2 are processed to have flat planes precisely parallel to each other. On the upper surface of the holding plate 2, a top ring 3 is provided as a loading member for giving a pressing force. The top ring 3 is attached to the lower end of a rotating shaft 4 and is driven to rotate at a predetermined rotational speed by a driving mechanism (not shown) which is attached to the rotating shaft 4 while giving a predetermined pressing force downward.
Under the holding plate 2, a turn table 5 having an approximate disc-shape is horizontally disposed in parallel with the lower surface of the holding plate 2. The turn table 5 is rotatable on a rotating shaft 6 which is attached to the central portion of the lower surface of the turn table 5, at a constant rotational speed. The upper surface of the turn table 5 is processed to have a precise flat plane and a polishing pad 7 is adhered thereon. The polishing pad 7 is made of unwoven cloth of a visco-elastic body having a suitable toughness and flexibility. An abrasive slurry 8 including abrasive grains of colloidal silica is supplied onto the central portion of the upper surface of the polishing pad 7 by a pump 9. The reference numeral 10 denotes a drain receiver for receiving the used abrasive slurry and the reference numeral 11 denotes an abrasive slurry tank.
In such a polishing device, the wafer 1 is held to adhere on the lower surface of the holding plate 2 by using vacuum suction, wax-adhesion, or the like, and the temperature of the surface of the polishing pad 7 on the surface of the wafer 1 and the polishing condition, e.g., a relative speed between the wafer 1 and the polishing pad 7 and the like, are kept constant, in order to obtain a desired flatness of the wafer 1. Because the wafer 1 is sandwiched between the holding plate 2 and the turn table 5 through the elastic polishing pad 7, the portion of the wafer having a larger thickness receives a larger contact pressure than that of the portion having a smaller thickness, so that the portion having a larger thickness is preferentially polished. As the result, the surface of the wafer 1 becomes flat gradually as the processing progresses. Further, in order to obtain a higher flatness, it is required to make the surfaces of the holding plate 2 and the turn table 5 flat, to make the thickness of the polishing pad 7 uniform, to remove the heat of the surface of the polishing pad 7 caused by friction by flowing a cooling water through the inside of the turn table 5, and the like.
When the wafer 1 is polished by the above-described conventional method, removal of Si is carried out by the so-called mechano-chemical function which compounds a mechanical function and a chemical function. Therefore, there are a lot of factors which have an effect on flatness of the wafer. For example, they are the surface roughness, the characteristics of the surface of the polishing pad 7 such as a loading condition due to the removed material by polishing or the like, the characteristics of the body of the polishing pad 7 such as the thickness, the viscoelasticity or the like, the surface temperature of the polishing pad 7 which closely relates to a frictional heat generated by rubbing the wafer 1 with the polishing pad 7, the degree of the abrasive slurry 8 reaching the surface of the wafer 1, and the like.
The load applied to the polishing pad 7 varies with the position in the polishing pad 7 and is not uniform, for reason of its structure. The polishing pad 7 is influenced by the applied load. For example, at a portion thereof to which a larger load is applied, the surface roughness becomes higher (flatter) by friction caused by rubbing motion, much loading due to products by polishing are caused, and the polishing pad 7 is apt to become thin and hard by the applied pressure. That is, the ability of the polishing pad 7 to remove the material changes (ordinarily, it is degraded) and becomes non-uniform with respect to the portion of the polishing pad 7. The degree of such non-uniformity varies with time. There is also a problem that the cooling of the turn table 5 causes thermal deformation thereof.
That is, even if conditions to improve the flatness of the wafer 1 were established, practically, there was a limitation of flatness which could be obtained for the wafer 1 by thermal deformation of the turn table 5, wear of the polishing pad 7, non-uniformity of creep deformation, and change of them with the passage of time.